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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if (root == NULL) {
            return true;
        }
        return isSymmetric(root->left, root->right);
    }
    
    bool isSymmetric(TreeNode* l, TreeNode* r) {
        if (l == NULL && r == NULL) {
            return true;
        } 
        if (l == NULL || r == NULL) {
            return false;
        }
        if (l->val == r->val) {
            return isSymmetric(l->left, r->right) && isSymmetric(l->right, r->left);
        }
        return false;
    }
};